Soil food webs and ecosystem processes: Decomposition in donor-control and Lotka-Volterra systems

We analyze how populations and food web structure affect the decomposition process. Based on a theory of carbon dynamics at the ecosystem level and food web theory at the population level, a link between population- and ecosystem-level theories is generated through a series of factors connecting population parameters to ecosystem-level decomposition rates. These factors are the ratio of the steady state biomass of a functional group to the mass of its resource, the total mortality rate of a functional group, and a carbon recycling index defined for each living component in the decomposer system. The factors defined at the primary decemposer level embody information about the whole food web. Two different assumptions about the mechanisms of trophic interactions in the food web are used: donor-control and Lotka-Volterra. The results show how the structure of the soil community and the characteristics and properties of soil fauna contribute to the decomposition process. Under the assumption of donor-controlled trophic interactions, the addition of soil fauna will always increase decomposition rate (mass loss), and the decomposition rate increases with the number of trophic levels. However, the maximum increase in decomposition rate due to fauna is determined by microbial properties only. These results do not hold under the assumption of Lotka-Volterra trophic interactions, where the decomposition rate depends on whether the number of trophic levels is odd or even, interaction structure, and the input of detritus to the system.